1. Field of the Invention
This invention relates to a fluid filtration system for use in purifying water derived from ground water and surface supplies. More specifically, the invention pertains to a water filtration medium comprising comminuted manganese dioxide particles which have been treated with ferrous ions, as well as methods of making and using the medium.
2. Description of the Related Art
Water derived from ground water and surface supplies has always been "contaminated" with naturally occurring materials, such as hydrogen sulfide, iron, manganese, and certain naturally occurring heavy metals derived from the elemental content of the earth and from various biological reactions that occur. More recently these same contaminants, and many more, have been introduced into these sources of water by mining and industrial activities. For example, industrial waste solutions have introduced several heavy metal contaminants into potential sources for drinking water. Various effluents from nuclear processing plants have introduced, for example, uranium, radium, cobalt, barium, strontium and similar ions. Certain of these contaminants are also from naturally occurring sources. This increasing burden of contaminants, and a growing knowledge of the problems they produce, has gradually led to increases in the restrictions concerning the maximum levels of these contaminants at the "point of use" of the water. The Environmental Protection Agency has established the maximum acceptable concentration of these contaminants in drinking water, as listed in Table I.
Probably the most common medium for the treatment of water to reduce various contaminants is an ion exchange resin. Depending upon the ions involved, this can be an anionic resin, a cationic resin or even a mixed resin medium. Generally, the resin beds are not selective as to the ions absorbed. The treatment process typically involves a regenerative bed where the absorbed ions are removed so as to condition the resin for additional removal. However, with some of the above-described contaminants, ion exchangers lack any appreciable affinity or lack the necessary selectivity to remove only the trace impurities.
TABLE I ______________________________________ EPA DRINKING WATER STANDARDS MAXIMUM LEVEL PARAMETER MAXIMUM LEVEL mg/l ______________________________________ Arsenic 0.05 Barium 2.0 Cadmium 0.05 Chromium 0.10 Copper 1.3 (action level) Iron 0.3 Lead 0.015 (action level) Manganese 0.05 Mercury 0.002 Selenium 0.05 Silver 0.05 Sulfide 0.025 (Odor Threshold) Radium 5 pCi/l 0.01 Gross Alpha 15 pCi/l Gross Beta 4 millirem/yr Turbidity 1/TU Coliform Bacteria 1/100 ml ______________________________________
There are certain organic and inorganic adsorbents that have been utilized in water treatment in the past. One such organic adsorbent is granulated activated carbon. While carbon has exceptional removal capabilities for chlorine and certain organic contaminants, carbon beds lack usefulness for most heavy metals and are usually used in combination with some other treatment method. Bacterial growth is also very prevalent within the carbon bed.
Various zeolite are known for their selective affinities for certain metal ions. For example, "Greensand" (Ionac M-50 available from Sybron Corp.) is a zeolite that has been treated with potassium permanganate to impregnate the surface with a layer of the higher oxidative oxides of manganese. This material has been used specifically for the oxidation and filtration of sulfide, iron and manganese. It has a limited capacity due to having only a surface coating of active material before "breakthrough" occurs, so must be frequently regenerated. When the media is "exhausted", it must be regenerated with more permanganate.
Two other materials having possible use in water treatment are KDF-55D (available from KDF Fluid Treatments) and BIRM (available from Clack, Inc.). Both of these are very limited in performance and design options for the treatment of water sources with a variety of contaminants.
U.S. Pat. Nos. 4,551,254 and 4,581,219 describe a process in which powdered electrolytic manganese dioxide (EMD) of a certain type is immersed in an acidic solution of divalent manganese ions for several days. The mass that results is broken into pieces, classified to have a size of 20 to 48 mesh and neutralized. The patent states that this results in a packing that has high surface activity and macroscopic pores through which water is able to pass readily. The initial powder is said have microscopic porosity and is of gamma form. After processing the manganese dioxide is converted, according to the patent, into the beta form (or a mixture of the gamma and beta form). The macroporous nature of this product provides primarily filtration and not a catalytic or oxidation reaction to retain the contaminants. The patents report that the product was studied for the removal of low levels of manganese, iron and primarily chromaticity (colored matter) from lakes or rivers. Contact times reported in these references were about 2.5 minutes.
Japanese Patent Application No. 63-194710 (laid open Aug. 11, 1988), states that alpha type manganese dioxide was tested and reported to be a better media for the removal of low levels (generally below the EPA limits) of contaminants from water than the media reported in the Imada patents.
The above prior art dealing with electrolytic manganese dioxide teaches that there must be a conversion from an initial complete gamma form to another form (beta plus gamma, beta alone or alpha alone), and that a microscopic pore structure must be converted to a macroscopic pore media.
U.S. Pat. Nos. 5,082,570 and 5,078,889 describe a manganese dioxide adsorption medium for heavy metals which is derived from electrolytic manganese by a sequence of comminuting, sizing, neutralizing, washing and drying steps. This material appears to have a capacity for lead which approaches that of colloidal hydrous manganese dioxide with a reported capacity of 0.65 milligrams of lead absorbed per gram of medium. The material, however, still has problems. The problems include a bleed off of extremely fine suspended solids in the early stages of use, a low capacity, and reproducibility of performance.
There is, accordingly, a continuing need for a water filtration medium that exhibits a higher adsorptive capacity for drinking water contaminants, especially lead; a reduction in bleeding and solubility in the initial exposures; a more reproducible performance; the ability to use a wider range of particle sizes of manganese dioxide; a more effective oxidation of iron, manganese, and hydrogen sulfide; and a more effective oxidation of organics.